The invention provides for a thermoelectric system comprising a substrate comprising a first complex oxide, wherein the substrate is optionally embedded with a second complex oxide. The thermoelectric system can be used for thermoelectric power generation or thermoelectric cooling.
|
1. A thermoelectric system, comprising:
(A) a substrate comprising a first complex oxide doped with La, Nb or Y, and
(B) a second complex oxide;
wherein the second complex oxide is
(a) embedded within the first complex oxide or
(b) a first layer indirectly over one surface of the first complex oxide wherein the first layer does not contact the first complex oxide;
wherein: the second complex oxide is of the formula M1a-xM2bM3xOc-δ(I) or M1aM2b-xM3xOc-δ(II) or M1aM2bOc-δ(III); M1 is one element selected from the group consisting of Na, K, Li, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Pb, Ca, Sr, Ba, Al, Y, and Bi; M2 is one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, W, Nb, Ta, and Ag; M3 is one element selected from the group consisting of La, Nb, and Y; M1, M2 or M3 are different elements from each other; and 1≦a≦4; 1≦b≦4; 2≦c≦10; 0≦x≦4; and 0<δ≦c for formula (I) and (II) and 0≦δ≦c for formula (III).
14. A thermoelectric system, comprising:
(A) a substrate comprising a first complex oxide doped with La, Nb or Y, and
(B) a second complex oxide;
wherein the second complex oxide is
(a) embedded within the first complex oxide,
(b) a first layer directly over one surface of the first complex oxide, or
(c) a first layer indirectly over one surface of the first complex oxide wherein the first layer does not contact the first complex oxide;
wherein: the second complex oxide is of the formula M1a-xM2bM3xOc-δ (I) or M1aM2b-xM3xOc-δ (II); M1 is one element selected from the group consisting of Na, K, Li, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Pb, Ca, Sr, Ba, Al, Y, and Bi; M2 is one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, W, Nb, Ta, and Ag; M3 is one element selected from the group consisting of La, Nb, and Y; M1, M2 or M3 are different elements from each other; and 1≦a≦4; 1≦b≦4; 2≦c≦10; 0<x≦4; and 0<δ≦c for formula (I) and (II).
2. The system of
3. The system of
4. The system of
5. The system of
6. The system of
7. The system of
8. The system of
10. A device comprising the system of
12. The system of
15. The system of
16. The system of
17. The system of
18. The system of
19. The system of
21. The system of
22. The system of
23. The system of
25. A device comprising the system of
|
This application claims priority as continuation application to PCT International Patent Application Ser. No. PCT/US2008/055720, filed on Mar. 3, 2008, which claims priority to U.S. Provisional Patent Application Ser. No. 60/892,820, filed on Mar. 2, 2007, respectively; which are hereby incorporated by reference.
The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The government has certain rights in this invention.
This invention relates generally to complex oxides useful for thermoelectric energy conversion.
Solid-state thermoelectric systems that can convert thermal energy to electrical energy directly are very attractive because they can be operated with various types of heat energy sources and they are environmentally benign. Furthermore, the systems can be operated in refrigeration, heat pump, or power generation mode, and they can be made either very large or miniaturized because of leg-type structures that are simpler than those of conventional systems that require hazardous working fluid or complicated moving parts. Consequently, thermoelectric systems are extremely promising for a variety of applications including, for example, cooling systems for the local heating in microprocessors and optical components, and power generation systems that recover waste heat from conventional power plants and in hybrid automobiles.
However, the conversion efficiencies for current thermoelectric materials based on bismuth telluride and/or semiconductors are not high enough to make them economically viable as compared to current energy conversion systems. In addition, popular thermoelectric materials tellurium and bismuth are toxic and hazardous. Thus, it has been impossible for current thermoelectric systems to become widely used.
With worldwide energy and climate crises on hand, research into high-Z thermoelectric materials has re-emerged with utmost importance. As energy prices rise and thermoelectric efficiency improves, the prospect of thermoelectric energy conversion devices becoming widespread is now on the horizon. In addition, heat transport from microprocessors is becoming more often the limiting factor in improving performance. Thermoelectric Peltier devices could play a significant role in upholding Moore's law in the near future.
Many recent studies on the improved properties of nanoscale thermoelectric materials have offered new options in engineering the thermoelectric properties S, σ, and κ, though few if any offer any fundamental understanding that may lead to bulk devices with ZT>5. While the present state-of-the-art, Bismuth Telluride, is well understood, materials such as strontium titanate with much higher known thermopowers offer both a non-toxic platform from which to engineer a higher ZT and a pathway to understanding thermoelectricity in more complex material systems.
As far back as 1967, a study on the transport properties of bulk strontium titanate (STO) and several of its doped forms revealed that oxygen-deficient strontium titanium oxide (STO) achieved a thermopower S as high as 800 mV/K. Other work has revealed that Lanthanum-doped STO (SLTO), such as having the formulae Sr1-xLaxTiO3, has a lower thermopower (˜300 mV/K at low La concentration, decreasing with added La) but much higher electrical conductivity σ and so can achieve higher ZT (˜0.1 at 300K). In addition, a recent study by H. Ohta et al. has brought yet new attention to the thermoelectric properties of STO, in which is attributed a “giant” thermopower to a 2DEG at a rutile-TiO2/STO interface, though the overall ZT of this device is low due to the low conductance of the interfacial layer compared with that of the surrounding substrate and thin-film.
U.S. Pat. Nos. 6,727,424 and 7,291,781 and U.S. patent Application Pub. Nos. 2006/0037638, 2003/0168641, and 2002/0037813 disclose certain complex oxides having certain thermoelectric properties.
The invention provides for a thermoelectric system, comprising a substrate comprising a first complex oxide. The substrate can further comprise a second complex oxide (a) embedded within the first complex oxide or (b) is a first layer directly or indirectly over one surface of the first complex oxide. The second complex oxide can be any suitable nanostructure embedded within the first complex oxide.
A suitable first complex oxide for use in the present invention is of the formula M1a-xM2bM3xOc-δ or M1aM2b-xM3xOc-δ; wherein M1 is one element selected from the group consisting of Na, K, Li, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Pb, Ca, Sr, Ba, Al, Y, and Bi; wherein M2 is one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, W, Nb, Ta, and Ag; M3 is one element selected from the group consisting of La, Nb, and Y; M1, M2 or M3 are different elements; and 1≦a≦4; 1≦b≦4; 2≦c≦10; 0≦x≦4; and 0<δ≦c. The first complex oxide is oxygen-deficient, or oxygen-deficient and doped. Suitable first complex oxides include, but are not limited to, oxygen-deficient, or oxygen-deficient and doped, BaTiO3, SrTiO3, PbTiO3, NaCo2O4, and BiFeO3. In some embodiments of the invention the first complex oxide has a ZT≧1.5 at 300 K and/or ZT≧15 at 125 K.
A suitable second complex oxide for use in the present invention has the formula of the first complex oxide as defined above, or is of the formula M1aM2bOc-δ; wherein M1 is one element selected from the group consisting of Na, K, Li, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Pb, Ca, Sr, Ba, Al, Y, and Bi; wherein M2 is one element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Mo, W, Nb, Ta, and Ag; M1 is not the same element as M2; 1≦a≦4; 1≦b≦4; 2≦c≦10; and 0≦δ≦c. Suitable second complex oxides include, but are not limited to, BaTiO3, SrTiO3, PbTiO3, NaCo2O4, and BiFeO3.
Suitable La for the use in the invention are Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.
The thermoelectric system can further comprise a LaAlO3 (LAO) layer between the substrate and the first layer. The LAO layer can be of a thickness of less than about 20 nm.
The present invention also provides for a device comprising the thermoelectric system of the present invention, wherein the first complex oxide substrate contacts a first electrode and a second electrode. The present invention also provides for a method of creating an electric current comprising: providing such a device, and increasing the temperature of the first electrode, such that an electric current is created that flows from the first electrode, through the substrate, and to the second electrode. The present invention provides for a thermoelectric power generator or thermoelectric cooler comprising such a device.
The present invention further provides for a composition comprising a lanthanum-doped strontium titanate having the formula Sr1-xLaxTiO3-δ, wherein 0<x<1, and 0≦δ<3. In some embodiments of the invention, 0<x<0.04 and/or 0<δ<0.04.
The foregoing aspects and others will be readily appreciated by the skilled artisan from the following description of illustrative embodiments when read in conjunction with the accompanying drawings.
Before the present invention is described, it is to be understood that this invention is not limited to particular embodiments described, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.
Where a range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the invention. The upper and lower limits of these smaller ranges may independently be included or excluded in the range, and each range where either, neither or both limits are included in the smaller ranges is also encompassed within the invention, subject to any specifically excluded limit in the stated range. Where the stated range includes one or both of the limits, ranges excluding either or both of those included limits are also included in the invention.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. All publications mentioned herein are incorporated herein by reference to disclose and describe the methods and/or materials in connection with which the publications are cited.
It must be noted that as used herein and in the appended claims, the singular forms “a”, “and”, and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a layer” includes a plurality of such layers, and so forth.
These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the invention as more fully described below.
A measure of the efficiency of a thermoelectric material can be described by the thermoelectric figure of merit (often called as ZT), which is defined as S2σ/k, where S, σ, and k are thermopower (or Seebeck coefficient), electrical conductivity, and thermal conductivity, respectively (see
SrTiO3 is an insulator. La-doping with La-doping with oxygen (O2−) vacancies of SrTiO3 results in a metallic or degenerate semiconductor state of Sr1-xLaxTiO3-δ. La-doping changes Ti4+ to Ti3+ and releases one electron. The creation of an oxygen vacancy releases two electrons. By combining La-doping with oxygen (O2−) vacancies, we show here that Sr1-xLaxTiO3-δ grown in thin-film form on STO single-crystals can improve the overall thermoelectric properties of this material system by over a factor of ten. In addition, enormous power factors can be achieved in these thin-film materials which may offer a pathway to high-ZT. With worldwide energy and climate crises on hand, research into high-Z thermoelectric materials have re-emerged with utmost importance. As energy prices rise and thermoelectric efficiency improves, the prospect of thermoelectric energy conversion devices becoming widespread is now on the horizon. In addition, heat transport from microprocessors is becoming more often the limiting factor in improving performance. Thermoelectric Peltier devices could play a significant role in upholding Moore's law in the near future.
Complex oxides have been found to be the best candidates for widespread deployment of thermoelectric systems for one or more of the following reasons: non-toxic, inexpensive, high efficiency, wide tunability in thermoelectric properties, and large effective mass of electrons, which enhances thermopower.
In one embodiment, SrTiO3 is used as a model for a wide range of complex oxides. By doping with lanthanum and intentionally making oxygen vacancies, the electrical conductivity of SrTiO3 can be significantly enhanced. Meanwhile, the oxygen vacancies also play an important role in increasing thermopower as well as reducing the thermal conductivity by scattering. Furthermore, the optimization of the electrical conductivity, thermopower, and thermal conductivity for better thermoelectric performance using the above mentioned technique lead a significant enhancement in thermoelectric figure-of-merit.
In one embodiment, highly efficient thermoelectric materials are developed from complex oxides by tuning material properties. It is usually difficult to increase thermoelectric figure-of-merit due to the strong correlation between three thermoelectric properties, i.e, electrical conductivity, thermopower, and thermal conductivity. In order to achieve high figure-of-merit, it is required to obtain high electrical conductivity, high thermopower, and low thermal conductivity. However, it has been very challenging to achieve them at the same time due to the intrinsic nature of electron transport. For example, when the electrical conductivity increases, themopower decreases and thermal conductivity increases. In one embodiment, electrical conductivity and thermopower increase at the same time by doping complex oxides with lanthanum and introducing oxygen vacancies. In addition, the thermal conductivity is also reduced by scattering due to the defects and impurities created by the doping, the vacancies, and a low temperature growth process. Thus a high thermoelectric-figure-of merit can be achieved, overcoming many drawbacks of current state-of-the-art thermoelectric materials and conventional energy conversion systems as described above.
In one embodiment extremely high power factors were achieved when SLTO was grown on an STO substrate. Extreme care was taken to ensure that contributions from the underlying STO did not convolute the measurement of the SLTO film properties; in fact, it was determined that the STO substrate almost always played a role when an SLTO film is grown directly on it. It has recently been shown that oxygen vacancies formed during film growth in STO can convolute transport properties on interfaces, likewise, this can occur for resistivity and thermopower measurements on our thin-films. In-situ across-sample resistance measurements during film growth confirmed that it is in fact the film itself which activates the reduction of the STO substrate. STO annealed under identical thermal and temporal conditions to those during film growth, but without film growth, will not reduce appreciably. However, growing films at pressures lower than 10−4 Torr at elevated temperatures reduces the substrate, which is optically darker than non-reduced STO upon completion of the growth process and can have a backside sheet resistance Rs as low as several ohms.
In one embodiment, to mitigate the reduction of the STO substrate during film growth and in turn diminish the substrate contribution to transport measurements, SLTO films were grown at 450° C. A low oxygen partial pressure of pO2=2×10−7 Torr (except in the case of a varied-pressure series of samples) was used to ensure the films were oxygen-deficient. Electron energy-loss spectra (EELS) of a typical SLTO/STO film/substrate (
In one embodiment, to reduce or eliminate any contribution from the substrate, a thin (5-20 nm) LAO layer was grown beneath the SLTO film via Laser-MBE to prevent oxygen diffusion out of the STO. LAO was effective in blocking oxygen diffusion since both Lanthanum and Aluminum can only exist in the 3+ oxidation state, whereas Ti in STO can readily go from 4+ to 3+ and in turn electrostatically allow the presence of oxygen vacancies.
SLTO growth on a layer of LaAlO3 (LAO) that was <20 nm in thickness on an STO substrate was necessary to achieve films with good thermoelectric properties. The thickness of the LAO layer was chosen so as to be thick enough to prevent oxygen diffusion while still thin enough for the LAO to be fully strained to the STO substrate, allowing the SLTO films to remain quasi-homoepitaxial and high-quality. RHEED oscillations were seen with good intensity for the duration of SLTO film growth on <20 nm LAO, indicating highly crystalline films growing two-dimensionally.
Measurements such as spreading probe or c-v show that oxygen diffusion out of the substrate does not increase the conductivity below the film appreciably. Using an LAO underlayer thus effectively isolates the SLTO film during transport measurements and allows for accurate determination of the film's thermoelectric properties.
The thermopower S of samples grown at various thicknesses with and without an LAO underlayer and on various substrates has been measured. A measured thermopower on the film surface is a sum of the thermopowers in the layers of the structure underneath (resistors in parallel), weighted by their respective conductances G=1/Rs. Although the LAO/STO interface is a known high-mobility interface, the Rs value of this interface (6500Ω/is still very large compared to that of the SLTO film (on the order of 100Ω/
and so will have a negligible contribution to the thermopower. On a control sample of 10 nm LAO on STO, we measured S=0.47 mV/K, with an Rs of 700 kΩ/
Likewise, a substrate Rs of ˜100 MΩ/
as measured on the backside (in addition, add spreading probe or CV data here) indicates that the substrate contribution to thermopower is almost zero. It may be that both any LAO/STO interface effects and the substrate will not significantly convolute the transport measurements of the SLTO films, Assuming a thermopower in the bulk STO substrate to be a maximum of 0.8 mV/K1, the thermopower of the film can be extracted from the measured thermopower and will differ by at most ˜1% from the measured value.
The electrical resistivity of the SLTO films can likewise be convoluted in measurement due to the large conduction volume of the substrate despite its relatively low resistivity. During a 4-point measurement the film/substrate system may be effectively modeled as resistors in parallel, and a resistance of roughly one order of magnitude greater in the substrate than the film introduces less than 5% measurement error into the resistivity measured from the sheet resistance RS. A resistance difference of several orders of magnitude was achieved through the use of the LAO underlayer, as was confirmed by spreading probe and by grinding off ˜50 μm or more of the film plus substrate and measuring RS this far below the film.
When shows thermopower is plotted as a function of Oxygen growth pressure, it seems that as more oxygen vacancies are added to the film, both electrical conductivity and thermopower increase to give very high power factors.
The c-axis lattice parameter of the films is distorted relative to that of the bulk (SLTO film c=3.9460 Å, a=3.9057 Å; STO bulk c=a=3.9056 Å), and x-ray diffraction (XRD) shows that all films grown under the presented conditions have a high degree of crystallinity (SLTO 002 full-width-at-half-maximum=0.125°) out to thicknesses as high as 1000 nm. Additionally, high-resolution transmission electron microscopy (HRTEM) verifies that the SLTO films are homoepitaxial with the STO substrate.
Films were also grown on substrates of (La0.18Sr0.82)(Al0.59Ta0.41)O3 (001), LaAlO3 (100), DyScO3 (100), NdGaO3 (110), and STO/Si (100), with lattice parameters of 3.79 Å, 3.86 Å, 3.94 Å, and (110Si)=3.84 Å, respectively. The measured thermopower of films on these substrates were at best half that of those on STO or STO/LAO, and film quality was also considerably worse as indicated by broader XRD film peaks and the presence of other cubic phases with similar lattice parameters.
The thermoelectric system of the present invention can be synthesized in any suitable method. Such methods include the methods described in Example 1 and 2 described herein.
The device of the invention can be a thermoelectric power generator or thermoelectric cooler. The device of the invention can be used for thermoelectric power generation or thermoelectric cooling, such as for computer chip cooling.
The invention having been described, the following examples are offered to illustrate the subject invention by way of illustration, not by way of limitation.
SrTiO3 (STO) is a cubic perovskite (Pm3m) with a bulk lattice parameter of 3.9056 Å. Out-of-plane lattice parameters of the films were characterized via x-ray diffraction using a Panalytical X'Pert MRD Pro 4-circle diffractometer for θ-2θ and rocking curve (ω) scans. All SLTO films grown were slightly distorted (˜1%) with the c-axis measuring 3.946 Å for typical 100 nm thick films.
TEM characterization and electron energy-loss spectroscopy (EELS) were performed on an FEI Tecnai F20 equipped with a high-angle annular dark-field (HAADF) detector. The quasi-homoepitaxial films (αSLTO bulk=3.9057 Å) exhibited smooth surfaces and flat interfaces with the substrate, and maintained uniform thicknesses through the entire specimen. STEM EELS was performed across the SLTO/STO interface with an energy resolution of 0.7 eV.
In addition, RHEED pattern intensity oscillations of the STO 001 peak were visible throughout the growth of films to thicknesses of nearly 200 nm.
Targets, Substrates, PLD
Pulsed-laser deposition (PLD) targets were stoichiometric and provided by Praxair . . .
Both STO (100) single-crystal substrates for PLD growth and Sr0.98La0.02TiO3 (100) single-crystals for bulk studies were supplied by Crystec.
Films were grown via pulsed-laser deposition (PLD) or Laser-molecular beam epitaxy (Laser-MBE) at a 10 Hz laser repetition rate with energy density of ˜2 J/cm2 for 10 minutes in the case of PLD-grown samples, and 1 Hz at 1.8 J/cm2 for 60 minutes in the case of Laser-MBE-grown samples.
Tuna
Nanoscale conductivity measurements were performed on a Digital Instruments Nanoscope-IV Multimode AFM equipped with a conductive-AFM application module (TUNA{TM}). Using conductive AFM tips it is possible to measure local electrical and topographical properties both simultaneously and independently. The investigations were performed with commercially available Pt-coated Si-tips (MikroMasch) and nitrogen doped diamond coated Si-tips (NT-MDT). Current amplification settings of the C-AFM equipment of 1V/pA and 10V/pA at an applicable voltage range of +−12V were used. For a typical scan rate of 0.5 to 1.0 microns per second, the noise level was of the order of 50 fA at a bandwidth of 250 Hz. All data were acquired under ambient conditions and at room temperature. Cross-sectioned samples were produced by cutting in half the original 5×5 mm sample and gluing the halves together with a cyanoacrylate-based glue (Superglue). Local surface roughness below 3 nm rms was achieved by dry mechanical polishing using diamond lapping film.
Thermoelectric Measurements
Because STO is a well-known thermoelectric with high thermopower, deconvoluting the transport in the substrate from the transport in the film must be done carefully and thoroughly. To facilitate this, transport (4-point Van der Pauw sheet resistance and thermopower) was measured on both the film and backside of the substrate after film growth for every sample.
A laser molecular beam epitaxy (MBE) is used to deposit 50-nm-thick Lanthanum-doped SrTiO3 films from a target containing SrO, TiO2, and La2O3 on 5 mm×5 mm×500 μm single-crystalline SrTiO3 substrates with a 10-nm-thick LaAlO3 buffer layer. During the deposition, the substrate is kept at a temperature and a vacuum pressure of 450° C. and ˜10−7 Torr, respectively. The sample is cooled at about 10° C./min and the pressure is maintained throughout the entire process. The laser energy density, the repetition rate, and the deposition rate are ˜1.6 J/cm2, and 1 Hz, respectively. The vacuum pressure is high enough to produce oxygen-deficient, Sr1-xLaxTiO3-δ (δ>0) films. The substrate is prepared by depositing a fully strained 10-nm LaAlO3 buffer layer on TiO2-terminated (001) SrTiO3 at 850° C. and ˜0.75×10−4 Torr. When the substrate is cooled after the LaAlO3 deposition, oxygen is purged into the chamber till atmospheric pressure. This minimize the oxygen vacancies that might be created during the buffer layer growth. The LaAlO3 buffer layer plays an important role in preventing oxygen diffusion considerably during the film growth since the LaAlO3 barely reduced due to the invariance of 3+ state of aluminum. When a Sr1-xLaxTiO3-δ film is deposited directly on a bare SrTiO3 substrate, the oxygen in the substrate is strongly reduced. The back side of the substrate is very conductive even after the film is completely removed. The oxygen-deficient film is responsible for the substrate reduction. When bare SrTiO3 is annealed without a film deposition under the same temperature and pressure as those of the film growth process, no such electrical conductance in the substrate is observe. The relatively low growth temperature can reduce activation energy for oxygen diffusion out of a substrate so that substrate reduction is minimize. In addition, low temperature growth creates defects in Sr1-xLaxTiO3-δ films, which result in suppression of thermal conductivity.
In-plane electrical property measurements are performed in a vacuumed variable-temperature cryostat after a thin layer of chromium/gold films is evaporated on the four corners of the film. During the metal evaporation, chamber pressure is maintained at ˜10−6 Torr in order to avoid oxidizing the film. Current-voltage sweep measurements indicate that the contacts between the metal and the oxide film are ohmic. The Van der Pauw method (W. R. Runyan, T. J. Shaffner, Semiconductor Measurements & Instrumentation (McGraw-Hill, New York, ed. 2, 1998)) is used to obtain electrical resistivity (p=1/σ) from sheet resistance (Rs) measurement. The resistivity can be calculated from the relation, ρ=Rs·t, where t is the thickness of a conducting layer. For thermopower measurement, the sample is mounted between two thermoelectric devices, and T-type thermocouples on both edges of the sample. The thermoelectric voltage generated across the sample is recorded using the copper leads of the thermocouple as the temperature difference between the two edges is increased from 0 to ±5˜10 K. The thermopower is determined by the linear fitting of the thermoelectric voltage as a function of the temperature difference. The thermopower of Copper is ˜1.83 μV/K at 300 K and is no greater than a few μV/K for our measurement temperature range of 20-400 K (D. M. Lowe, Ed., CRC Handbook of Thermoelectrics (CRC Press, Boca Raton, Fla., 1995), pp. 390-391). The thermoelectric voltage across the copper leads generated from ˜10 K temperature difference is assumed to be negligible compared with measured thermoelectric voltages up to several mV.
The substrate influences the measurement. Care is taken in finding thin film properties due to the contribution from the substrate that is usually several orders of magnitude thicker than the film. This implies that the sheet resistance of the film and the substrate could be comparable even though the resistivity of the substrate is several orders higher than that of the film. For example, Rs,film from ρfilm=5 mΩ-cm and tfilm=50 nm is comparable to Rs,substrate from ρsubstrate=2000·ρfilm=104 mΩ-cm and tsubstrate=100 μm. This substrate resistivity is easily reached by a slight doping of ˜1017 carriers per cm−3, which needs only ˜10−4% Oxygen vacancy if mobility falls ˜6 cm2/V-s (Okuda (2001)). In order to subtract the substrate contribution, three layers—the film, the insulating buffer layer, and the substrate—are connected in parallel, and electron transport occurred only through the film and the substrate. The length of the sample is substantially longer than the thickness of the sample. The film properties are measured using the sheet resistance and the thermopower before and after removing the film. The resistivity of the film, ρfilm is obtained from the relation, tfilm·(1/Rs,hetero−1/Rs,substrate)−1. Similarly, the thermopower of the film, Sfilm is expressed as Rs,film·[Shetero(1/Rs,film+1/Rs,substrate)−Ssubstrate/Rs,substrate] where Rs, hetero and Shetero are respectively the sheet resistance and the thermopower due to both the film and the substrate. This method has been used for determining the electrical properties of heterostructures (T. Koga, S. B. Cronin, M. S. Dresselhaus, J. L. Liu, K. L. Wang, Applied Physics Letters 77, 1490 (2000); H. W. Hillhouse, M. Tuominen, Microporous and Mesoporous Mater. 47, 39 (2001); H. Ohta et al., Nature Materials 6, 129 (2007)).
A mixture of fluorine and chlorine based acids is used remove the film. The crystallinity of the film is ensured by performing X-ray diffraction analysis as the film is etched.
The resistivity of a Sr1-xLaxTiO3-δ film (
The thermopower of the film is plotted in
where h, kB, and η are Planck and Boltzmann constants, and the reduced chemical potential. The degeneracy, z is 6. The carrier scattering parameter, r is defined as τ=τ0εr-1/2, where τ is relaxation time (V. I. Fistul, Heavily Doped Semiconductors (Plenum, New York, 1969)). When r=2.2 from μ(T)˜T−2.7, the effective mass is ˜16.7·m0 at 300 K.
The eminent thermopower near 150 K. Similar spikes in thermpower have been observed for bulk SrTiO3 and Nb-doped SrTiO3 due to phonon drag (Frederikse (1964); Okuda (2001); Ohta (2007)). Ohta et al. (2007) show that the peak temperature moves towards higher temperature as carrier concentration increases. In addition, large thermopower, ˜1 mV/K and ˜0.49 mV/K at 300 K are obtained from a heterointerface of TiO2/SrTiO3 and from one unit cell of SrTi0.8Nb0.2O3 confined by SrTiO3. Incidentally, the peak temperature of the SrTi0.8Nb0.2O3 is close to that of the film in question. Whether these films have such interfaces, they do not significantly affect the properties of relatively thick film (˜50 nm) due to the extremely small thickness of the interface (˜0.39 nm). For example, despite low electrical resistivity (˜0.43 mΩ-cm) calculated from carrier concentration of 2.4×1021 cm−3 in the SrTi0.8Nb0.2O3, the sheet resistance is ˜11 kΩ. This resistance is considerably larger than that of our film. In this case, the most of electron transport occurs through less resistive paths—the film rather than the interface. It is also evident that the interface effect would be negligible from the relation for ρfilm and Sfilm as described herein.
The out-of-plane thermal conductivity of 50˜500-nm-thick Sr1-xLaxTiO3-δ films is measured since there is no readily available measurement technique for the in-plane thermal conductivity of a thin film. It is reasonable to consider that the in-plane and the out-of-plane thermal conductivity are similar due to the homogeneity of the single-crystalline film and substantially shorter phonon mean free path than the thickness of the film. The length of the mean free path is estimated to be less than ˜1.6 nm at room temperature as sound velocity is ˜8000 m/s of a bulk SrTiO3. A differential 3 ω method (D. G. Cahill, Rev. Sci. Instr. 61, 802 (1990); T. Borca-Tasciuc, A. R. Kumar, G. Chen, Rev. Sci. Instr. 72, 2139 (2001)) and a thermoreflectance method (D. G. Cahill, Review of Scientific Instruments 75, 5119 (2004)) are used. The 3ω method assumes that heat does not spread in a film along the in-plane direction. This suggests that the thermal conductivity of a substrate should be much larger than that of a film. In order to comply this requirement, two films grown on Silicon substrates with a 10-nm-thick SrTiO3 buffer layer deposited epitaxially by using MBE are prepared (Z. Yu et al., Journal of Vacuum Science & Technology B 18, 1653 (2000)). A slow cooling rate of less than 1° C./min for the Silicon substrate samples is used to avoid deteriorating the film quality due to thermal expansion coefficient mismatch. Five different samples—a stoichiometric bulk SrTiO3, 500-nm-thick Sr1-xLaxTiO3-δ films on Silicon substrates grown at 650° C. and 450° C., 100- and 50-nm-thick Sr1-xLaxTiO3-δ films on SrTiO3—are used.
The lanthanum and oxygen vacancies suppress thermal conductivity as well as increase the power factor (i.e., both enhance ZT). As shown in
Finally, ZT is plotted as a function of temperature as shown in
In summary, a large thermoelectric figure-of-merit (ZT) is measured in oxygen-deficient lanthanum-doped single-crystalline strontium titanate (SrTiO3) thin films. ZT is ˜1.5 at 300 K and ramps up as temperature decreased, and reaches up to ˜17 at ˜125 K. The huge enhancement is due to the synergistic effects of doping lanthanum and creating oxygen vacancies—increase the power factor as well as decrease thermal conductivity. A large thermopower is attributed to oxygen vacancies while lanthanum doping and oxygen vacancies decrease electrical resistivity. Furthermore, the impurities are most responsible for a large suppression in thermal conductivity.
While the present invention has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation, material, composition of matter, process, process step or steps, to the objective, spirit and scope of the present invention. All such modifications are intended to be within the scope of the claims appended hereto.
Ramesh, Ramamoorthy, Majumdar, Arunava, Yu, Choongho, Scullin, Matthew L., Huijben, Mark
Patent | Priority | Assignee | Title |
10680153, | Mar 31 2016 | Murata Manufacturing Co., Ltd. | Thermoelectric conversion element and method for manufacturing thermoelectric conversion element |
10847703, | Jun 09 2015 | Murata Manufacturing Co., Ltd. | Thermoelectric conversion element, thermoelectric conversion module, and electrical device |
11164976, | Apr 08 2019 | KEPLER COMPUTING INC | Doped polar layers and semiconductor device incorporating same |
11289607, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11289608, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11296228, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11349031, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11355643, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11398570, | Apr 08 2019 | KEPLER COMPUTING INC | Doped polar layers and semiconductor device incorporating same |
11411116, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11417768, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11444203, | Apr 08 2019 | KEPLER COMPUTING INC | Doped polar layers and semiconductor device incorporating same |
11469327, | Apr 08 2019 | KEPLER COMPUTING INC | Doped polar layers and semiconductor device incorporating same |
11654635, | Apr 18 2019 | The Research Foundation for SUNY | Enhanced non-destructive testing in directed energy material processing |
11757043, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11837664, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11848386, | Apr 08 2019 | Kepler Computing Inc. | B-site doped perovskite layers and semiconductor device incorporating same |
11888066, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11888067, | Apr 08 2019 | Kepler Computing Inc. | B-site doped perovskite layers and semiconductor device incorporating same |
11908943, | Apr 08 2019 | Kepler Computing Inc. | Manganese-doped perovskite layers and semiconductor device incorporating same |
11916149, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11949017, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
11949018, | Apr 08 2019 | Kepler Computing Inc. | Doped polar layers and semiconductor device incorporating same |
Patent | Priority | Assignee | Title |
4401694, | Jan 22 1979 | Boeing Company, the | Method and apparatus for an optical sensor utilizing semiconductor filters |
5124310, | Aug 20 1990 | Energy Conversion Devices, Inc. | Laser ablation method for depositing fluorinated Y-Ba-Cu-O superconducting film having basal plane alignment of the unit cells deposited on non-lattice-matched substrates |
6727424, | Jul 18 2000 | Agency of Industrial Science and Technology | Complex oxide having high thermoelectric conversion efficiency |
7291781, | Mar 22 2002 | National Institute of Advanced Industrial Science and Technology | Complex oxide having n-type thermoelectric characteristics |
20020037813, | |||
20030168641, | |||
20040018409, | |||
20040232893, | |||
20050255315, | |||
20060021646, | |||
20060037638, | |||
20060102224, | |||
20060255310, | |||
EP494580, |
Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
Aug 11 2009 | The Regents of the University of California | (assignment on the face of the patent) | / | |||
Aug 11 2009 | MAJUMDAR, ARUNAVA | The Regents of the University of California | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 023468 | /0203 | |
Aug 12 2009 | YU, CHOONGHO | The Regents of the University of California | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 023468 | /0203 | |
Aug 14 2009 | HUIJBEN, MARK | The Regents of the University of California | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 023468 | /0203 | |
Aug 24 2009 | Regents of the University of California, The | Energy, United States Department of | CONFIRMATORY LICENSE SEE DOCUMENT FOR DETAILS | 023511 | /0377 | |
Oct 05 2009 | SCULLIN, MATTHEW L | The Regents of the University of California | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 023468 | /0203 | |
Nov 04 2009 | RAMESH, RAMAMOORTHY | The Regents of the University of California | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 023468 | /0203 |
Date | Maintenance Fee Events |
Aug 20 2015 | M3551: Payment of Maintenance Fee, 4th Year, Micro Entity. |
Aug 21 2015 | STOM: Pat Hldr Claims Micro Ent Stat. |
Aug 12 2019 | M3552: Payment of Maintenance Fee, 8th Year, Micro Entity. |
Sep 14 2022 | SMAL: Entity status set to Small. |
Mar 04 2024 | REM: Maintenance Fee Reminder Mailed. |
Aug 19 2024 | EXP: Patent Expired for Failure to Pay Maintenance Fees. |
Date | Maintenance Schedule |
Jul 17 2015 | 4 years fee payment window open |
Jan 17 2016 | 6 months grace period start (w surcharge) |
Jul 17 2016 | patent expiry (for year 4) |
Jul 17 2018 | 2 years to revive unintentionally abandoned end. (for year 4) |
Jul 17 2019 | 8 years fee payment window open |
Jan 17 2020 | 6 months grace period start (w surcharge) |
Jul 17 2020 | patent expiry (for year 8) |
Jul 17 2022 | 2 years to revive unintentionally abandoned end. (for year 8) |
Jul 17 2023 | 12 years fee payment window open |
Jan 17 2024 | 6 months grace period start (w surcharge) |
Jul 17 2024 | patent expiry (for year 12) |
Jul 17 2026 | 2 years to revive unintentionally abandoned end. (for year 12) |